The two most popular CSS preprocessors areSass and LESS, and they’re both available today in the CSS Editor on WordPress.comand the Jetpack plugin for self-hosted blogs. To use either language for your Custom CSS, just change the Preprocessor option before you save, and your Sass or LESS will automatically be compiled into CSS when it’s served to your site’s visitors.
If you’re not familiar with Sass or LESS, check out Getting Started with Sass or Using the LESS CSS Preprocessor for Smarter Style Sheets. You can ignore any instructions about installing the preprocessors or running them manually, since we handle that for you. Happy styling!